1. Field of the Invention
The present invention pertains generally to a composting system for biologically converting diverse and potentially hazardous source materials into safe and agriculturally beneficial soil amendments and fertilizers. More particularly, the invention comprises an array of enclosed containers which perform the various steps of composting, curing and biofiltration, and which provide only limited, controlled exchange of gases and liquids into the environment during the biological conversion process. In an even more specific embodiment, the source materials are first composted, then cured, and finally serve as biofiltration media in a closed-loop system. Gases are recirculated from compost through curing and biofiltration and back into the compost, while temperature and gas content within the closed loop are carefully regulated.
2. Description of the Related Art
Before about 1970, composting was typically a simple process in which waste materials were piled and allowed to sit until they decomposed. It was most frequently done on a small scale and was not often considered for industrial-scale problems. The ingredients placed into these piles were poorly controlled, and the resulting mixture would decompose unpredictably, frequently anaerobically, with strong odors associated therewith. Unfortunately, often the strength of these odors were in direct correlation to the loss of valuable fertilizer components such as nitrogen. Vermin were also often attracted to these piles, creating hazardous vectors for transmission of disease.
An advance in composting technology came from the realization that adding air to the composting mixture could increase the efficiency of composting. The microbes that produce more desirable fertilizer require air, and will smother inside of a static unaerated pile. Hence, the initial methods of aeration involved moving or agitating the compost to allow air into the stack. This method only partially satisfies the need for aeration, and consequently only poorly addresses odors and nutrient loss, and does nothing to limit access by vermin. composting mixture could increase the efficiency of composting. The microbes that produce more desirable fertilizer require air, and will smother inside of a static unaerated pile. Hence, the initial methods of aeration involved moving or agitating the compost to allow air into the stack. This method only partially satisfies the need for aeration, and consequently only poorly addresses odors and nutrient loss, and does nothing to limit access by vermin.
A typical example of this aeration is a windrow turner that picks up the compost and dumps it to one side. Most municipal composting sites are currently windrow turner operations, though process control is, unfortunately, quite primitive. Piles are typically turned at the convenience of the operator, rather than to optimize the composting process. A typical pile of compost will use all of its oxygen within about one-half hour, so such windrow turning is seldom related to actual oxygen demand. Turning is done seldom enough that microbes in the center of the pile are rapidly depleted, and the center of the pile stops composting. Turning the pile merely re-inoculates the center material with fresh microbes, and composting continues in the center of the pile for another one-half hour when the oxygen supply is, once again, depleted. Unfortunately, the repeated mechanical actions that are required for turning destroy some beneficial fungi that rely on large, filamentous growth. In addition to the oxygen and mechanical problems introduced by a windrow system, composting with windrow turners is typically done in an open, unsheltered area. The vagaries of weather and rainfall most often determine the water content of the composting mass. When there is too little rain, the pile is too dry. When there is too much rain, the pile is wet and requires frequent turning. Too much rain can also lead to problems with runoff of leachate. During the loss of leachate there will not only be a loss of fertilizer value but also a potential hazardous contamination of surrounding surface water and soil. In the open, of course, it is also very difficult to control access by vermin.
One method used to overcome some of the disadvantages of pile composting is to enclose compost piles in a building. An enclosure that keeps rain off of the compost allows better regulation of water content. However, such a facility is very expensive. Furthermore, with pile composting, various irritating and potentially toxic gases are sometimes produced. Since operators must enter the enclosure to maintain the composting process, enclosing compost also involves maintaining the quality of large volumes of air within the building. Without high-quality and high-quantity air handling systems, the atmosphere within an enclosure can be irritating, if not toxic, to an operator. Sadly, with the removal of air in the building is a removal of nutrients from the compost. Consequently, the resulting compost is little better in fertilizer value than the compost of the open windows and piles. These enclosed buildings do, however, help to control or prevent access to the compost by most vermin.
Some of the disadvantages of pile composting are overcome by more modem reactor vessel processes. By design, the reactor vessel is typically only slightly larger than the compost which it contains. This reduces the land area required to store the compost during the composting process. In addition to reduced land area, the total volume containing or enclosing the compost is also reduced. Lower total volume means reduced air handling requirements. Furthermore, in-vessel reactors also provide the opportunity for collection of potentially odorous emissions. The compost is enclosed, and exhaust air may be routed through a filtration system. This separation of operator from compost air benefits the health and safety of all operators. There are other benefits, beyond land space and air handling, from reactor vessels. Handling and mixing, which is required in all systems, can also be mechanized using reactor vessels, and the compost is enclosed.
Unfortunately, vessel systems to date are complicated systems which require precision construction techniques and permanent, stable foundations. This necessarily drives the cost of present reactor vessels systems to levels even higher than required for building-type enclosures. In exemplary prior art systems, organic waste is fed into an opening at one end of the reactor and compost is removed from the other end. The material is moved through the reactor by, for example, a complex moving floor apparatus or hydraulic ram. Aeration is sometimes provided by pressurized air forced through the organic waste from air vents located throughout the moving apparatus.
Some in-vessel systems also include mixing systems, typically rotating paddles or prongs, within the compost mass. Other in-vessel systems are static. The agitation systems used with in-vessel systems are expensive, prone to wear and failure, and provide agitation at intervals that are not readily controlled with respect to the progress of the composting process.
Even in the advanced in-vessel systems, there is still a limitation of the composting systems that must be addressed for wider acceptance in the marketplace. During the composting process, even in highly controlled in-vessel systems, there will always be a potential for generation of significant quantities of undesirable and odorous gases such as ammonia. Some artisans have reduced the levels of emissions of these gases through very careful measuring and control of the source materials which are undergoing biological transformation, but this control adds expense and undesirably limits the application of the composting system to only a very few applications. Other artisans have attempted to filter out of the gas stream the undesired contaminant gases. Various gas filtration devices have been proposed and implemented, including chemical scrubbers and biofilters. Chemical scrubbers tend to be quite expensive to operate, but more importantly produce new wastes that must be disposed of. This generation of secondary wastes tends to be very counterproductive. Furthermore, the waste removed in the form of contaminated gas or chemical scrubbers and filters represents a permanent loss of valuable soil nutrients.
Biofilters have more recently gained acceptance in treating odors from a diverse range of sources. These biofilters contain any of a fairly wide variety of substrate materials which support living organisms in an aqueous phase upon the surface of the substrate. These organisms feed upon the contaminants in the gas stream, and digest these contaminants into more basic and harmless components, such as carbon dioxide and water. While the substrate materials will only infrequently need replaced, they also may be used directly as an agricultural amendment of value and benefit. Consequently, a biofilter does not produce a second waste stream, but instead produces beneficial product for use in agriculture.
While biofiltration has enabled a compost facility to eliminate any secondary solid waste production, there has heretofore been very little control or regulation over the release of gases into the atmosphere. During rapid composting, it is possible to overload a biofilter with excessive levels of ammonia. Additionally, when source materials are introduced into the compost that are higher in nitrogen content than expected or for which the biofilter was designed, there will similarly be a surge in ammonia production. This surge can, on occasion, saturate the biofilter and lead to a release of undesirable levels of ammonia or other compounds into the environment. This not only presents an odor control problem, but also represents a loss of valuable nitrogen which would otherwise be most desirable for fertilizers used commonly with agricultural application. There is, therefore, a need to provide better control over the gases released from a compost system, while simultaneously allowing the compost system to handle a wider variety of source materials with less operator intervention.
In a first manifestation, the invention is a recirculating compost system having a closed aerobic compost vessel with a gas inlet and a gas outlet, a closed aerobic curing bin having a gas inlet and a gas outlet, and a closed biofilter having a gas inlet and a gas outlet. A gas stream is recirculated from the compost vessel through the biofilter to the curing bin and returned to the compost vessel. Ports are provided to introduce fresh air into the recirculation to replenish consumed oxygen, and to vent waste gas from the system. Additional biofiltration and monitoring are also available for further monitoring and control over the waste gas.
In a second manifestation, the invention is a method for biologically processing source materials into an agriculturally beneficial fertilizer. The steps include the recirculation of air and waste gases through a compost vessel, a biofilter and a curing bin. Various characteristics and composition of the gases are monitored and the system controlled in accordance therewith. The source material is cycled through the system from a compost vessel to a curing bin and finally to the biofilter. Additional controls and measurements are contemplated herein which enable the production of a fertilizer of consistent composition.
A first object of the present invention is to reduce the undesired loss of valuable soil nutrients from a composting system, and to consequently yield a higher value fertilizer than was heretofore possible. A second object of the invention is to enable precise measurement and control over the amount of contaminants released into the environment. A third object of the invention is to lower the cost of operation of a composting system, to make the system more economically attractive in the marketplace. Another object of the invention is to make a compost system more tolerant of variations in source material. A further object of the invention is to reduce the amount of operator intervention required to operate a compost system. Yet another object of the invention is to limit the type of intervention required, so that less technical training is required for an operator to successfully operate a compost system. These and other objects of the invention are achieved in the preferred embodiment, which will be best understood when considered in conjunction with the appended drawing figures.